1. Field of the Invention
This invention relates to the field of photonics, and in particular to a device for integrating demultiplexing and optical channel monitoring. The invention is applicable to integrated optical components, as well as components based on bulk-optics.
2. Description of Related Art
In an optical telecommunications network based on dense wavelength division multiplexing (DWDM), several closely-spaced wavelength channels are multiplexed onto a single optical fiber and transmitted to another system node where the channels are demultiplexed and detected individually. It is very important in DWDM networks to monitor the power in each wavelength channel, as well as other channel information, such as wavelength and optical signal to noise ratio (OSNR), which can vary significantly from channel to channel due to wavelength-dependent amplification or loss.
This monitoring is often performed at system nodes immediately before the main signal is demultiplexed. This is typically accomplished by tapping a small fraction of the signal (1% to 5%) off the main transmission line before the demultiplexer. These two signals, the “main” signal and the “monitoring” signal, are then sent to separate demultiplexers and separated into their individual wavelengths. The monitoring channels pass to a detector array for measuring the power in each channel, while the main signals split into individual optical fibers for coupling to their individual receivers or, for example, variable optical attenuators. This typical functionality is illustrated in FIG. 1. An input signal is input to an optical tap 1, which splits the signal into a first portion containing 97% of the light that goes to a demultiplexer 2 providing the optical outputs. The tap passes 3% of the light to a demultiplexer 3 that provides outputs that a detected by a detector array 4.
A key product focus for integrated optic and bulk-optic component manufacturers is integration of multiple functions into a single module. For example, it would generally be desirable to integrate an optical demultiplexer with an optical channel monitor, including the 1% optical tap. Such a device would accomplish the functionality illustrated in FIG. 1.
One major difficulty with this approach is that in integrated optics, optical taps often exhibit a polarization dependent loss (PDL), a major problem for all telecommunications components. Also, it is difficult to manufacture even a single optical demultiplexer on a single chip. The functionality demonstrated in FIG. 1 requires two demultiplexers. Furthermore, all of the components need to be individually temperature controlled. Integrating three building blocks into a common device results in a large device that is difficult to manufacture and has potential PDL problems.